1. Field of the Invention
The field of the present invention relates generally to I-F substitution loops for accurately measuring I-F signal levels, and more particularly relates to circuitry for reducing the phase noise occurring at the main summing junction of the substitution loop.
2. Discussion of the Relevant Art
Signal generator calibrators, such as the VM-24 manufactured by Weinschel Engineering Incorporated, USA, are used for measuring the amplitude of low level signals. The VM-24 signal generator calibrator includes a phase lock loop locked to a local oscillator for translating the input signal to be measured to a predetermined I-F frequency, and an I-F substitution loop for accurately measuring the level of the I-F signal. The range of measurement of the calibrator is limited by noise that may be inherent in the I-F signal. Accordingly, reductions in the noise within the I-F substitution loop offers extended low-level detection capability for the signal generator calibrator.
Many attempts have been made in the prior art to reduce the level of the noise occurring in electronic circuitry. For example, in Kerwin, et al U.S. Pat. No. 3,621,407 VARIOUS MULTI-LOOP RC active filter networks are disclosed for producing transfer functions having complex poles and/or zeros via use of at least two passive components and two voltage amplifiers, one having a positive gain which is less than unity and the other having a negative gain. In one embodiment shown in FIG. 10 of Kerwin, et al, a voltage amplifier having a positive gain K.sub.1 has an input terminal connected to a summing junction of a second voltage amplifier having a negative gain of magnitude K.sub.2, with the output of the positive gain amplifier 56 being fed back, via a passive element including an RC coupling network, to the summing junction, and with the output of the negative gain amplifier 54 being fed back to the summing junction through a distributed resistor of the passive element that is common to the coupling capacitor of the element connected to the output of the positive gain amplifier 56. The input signal is coupled via distributed capacitance of the passive element to the distributed resistor of the coupling to the summing junction. The circuitry provides a multi loop transfer function generating apparatus using the passive element of distributed active RC elements for providing a zero, at zero frequency, without requiring the use of a differential amplifier.
Nelson, et al U.S. Pat. No. 4,189,681, discloses an active filter in which two, second-order, multiple-feedback, active filter circuits are cascaded and the output of the second filter is fed back to the input of the first filter to realize a fourth-order filter for greatly attenuating signals outside the passband, while passing signals with very low phase shifting thereof that are within the passband.
Yokoyama U.S. Pat. No. 4,292,468, discloses a frequency selection circuit generally including an operational amplifier connected to receive an input signal at its non-inverting input terminal, a band rejection filter connected to an output terminal of the amplifier, a feedback resistor connected between the output terminal of the amplifier and its non inverting input terminal, and the output of the band rejection filter or bandpass filter being connected to a voltage follower amplifier, the output of the latter being connected, via feedback resistors, to both the inverting and non-inverting terminals of the operational amplifier. The circuit is equivalent to an LCR series resonant circuit, although not provided with inductance, the circuit can adjust resonant frequency, selectivity or quality factor Q, and circuit impedance at resonance, by varying the resistance in the circuit only.
A typical I-F substitution loop is disclosed in U.S. Pat. No. 4,219,770 which relates to insertion loss and a method of phase shift measurement.